Reject out-of-range column index in Dataset.add_formatter

AUTHORS

@@ -8,6 +8,7 @@ Here is a list of past and present much-appreciated contributors:
     Andrii Soldatenko
     Benjamin Wohlwend
     Bruno Soares
+    Charles Tonneslan
     Claude Paroz
     Daniel Santos
     Egor Osokin

src/tablib/core.py

@@ -638,7 +638,7 @@ def add_formatter(self, col, handler):
             else:
                 raise KeyError
 
-        if col is None or col <= self.width:
+        if col is None or col < self.width:
             self._formatters.append((col, handler))
         else:
             raise InvalidDatasetIndex

tests/test_tablib.py

@@ -19,7 +19,7 @@
 
 import tablib
 from tablib.core import Row, detect_format
-from tablib.exceptions import UnsupportedFormat
+from tablib.exceptions import InvalidDatasetIndex, UnsupportedFormat
 from tablib.formats import registry
 
 try:
@@ -603,6 +603,11 @@ def _formatter(cell_value):
         # Test once more as the result should be the same
         self.assertEqual(self.founders.dict, expected)
 
+    def test_formatter_invalid_column_index(self):
+        """Registering a formatter on an out-of-range index is rejected."""
+        with self.assertRaises(InvalidDatasetIndex):
+            self.founders.add_formatter(self.founders.width, str)
+
     def test_formatters_all_cols(self):
         """
         Passing None as first add_formatter param apply formatter to all columns.